Syrian warplanes have bombed a rebel headquarters near the Turkish border, missing their target but sending hundreds of Syrians fleeing across the frontier.

The attack on the Free Syrian Army base in Atima, two kilometres from the border, came a day before Turkish and Nato officials were due to start assessing where to station surface-to-air missiles close to the 900 km border.

Turkey, a major supporter of insurgents fighting to oust President Bashar al-Assad, has repeatedly scrambled jets along the joint border and responded in kind when Syrian shells have landed inside Turkey.

But Ankara, rejecting Syrian complaints that the Patriots were "provocative", stressed they would be used only to defend Turkish territory, not to create a no-fly zone inside Syria that rebels want to neutralise Assad's massive air power.

Describing the bombing of the FSA base, opposition activist Ahmed, who lives within a few blocks of it, said: "Two Syrian fighter jets came and fired five rockets. Three have hit farm areas and another two hit buildings near the base."

The overnight (NZ time) strike was one of the closest to the Turkish border carried out by Syrian jets. Ahmed said it was the first time they targeted the FSA base set up by senior rebel Mustafa al-Sheikh when he crossed over to Syria from Turkey two months ago.

Rebels fired anti-aircraft guns at the jets but they were flying too high to be hit, activists said. "I think the reason for the raid may have something to do with increased weapons movements (from Turkey)," Ahmed said.

Several hundred Syrians fled into Turkey after the raid and were being taken care of by the Turkish army. At least two wounded people were taken across the border.

The Turkish Anatolian news agency said an anti-aircraft shell fired during clashes in another Syrian border town, Harem, hit the roof of a house in the Turkish district of Reyhanli, causing no casualties.

After 20 months of conflict, rebels have been tightening their hold on farmland and urban centres to the east and northeast of Damascus, and have seized a string of military bases in the past 10 days.

PATRIOT DEPLOYMENT

A joint Turkish-Nato team will start work today (NZ time) assessing where to put Patriot missiles, how many will be needed and the number of foreign troops to be sent to operate them.

Turkey is reluctant to be drawn into the fighting, but the proximity of Syrian bombing raids to its border is straining its nerves. It is worried about its neighbour's chemical weapons, the refugee crisis on its border, and what it says is Syrian support for Kurdish militants on its own soil.

Activists say more than 40,000 people have been killed in Syria's civil war, which started with peaceful demonstrations for reform but grew into demands for the overthrow of 42 years of dynastic rule by Assad and his late father, Hafez al-Assad.

Attacks by mainly Sunni Muslim rebels against Assad's forces have become increasingly effective and deadly. The president, from Syria's Alawite minority which is linked to Shi'ite Islam, has responded with devastating artillery and air bombardment.

Hundreds of thousands of Syrians have fled their country and more than two million more have been displaced. The opposition said last week US$60 billion (NZ$73b) would be needed for reconstruction.

The military installations rebels have captured in the last 10 days include a major facility in the northern province of Aleppo and several bases around the capital Damascus.

EUPHRATES DAM CAPTURED

On Monday activists said rebels took control of the Tishreen dam on the Euphrates river, east of the city of Aleppo. Internet video footage showed gunmen inside what appeared to be the control room, undamaged following the rebel capture.

Other footage showed rebels opening up ammunition boxes, including one marked RPG (rocket-propelled grenades), which they said were seized from Assad's forces holding the dam.

On Sunday rebels said they had captured a helicopter base east of Damascus, their latest gain in a battle that is drawing nearer to Assad's seat of power in the capital.

The Marj al-Sultan base, 15 km from Damascus, is the second military facility on the outskirts of the city reported to fall to Assad's opponents this month. Activists said rebels destroyed two helicopters and taken 15 prisoners.

"We are coming for you Bashar!" a rebel shouted in an internet video of what activists said was Marj al-Sultan. Restrictions on non-state media meant it could not be verified.

The rebels have been tightening their hold on farmland and urban centres to the east and northeast of Damascus while a major battle has been under way for a week in the suburb of Daraya near the main highway south.

"We are seeing the starting signs of a rebel siege of Damascus," opposition campaigner Fawaz Tello said from Berlin. "Marj al-Sultan is very near to the Damascus Airport road and to the airport itself. The rebels appear to be heading toward cutting this as well as the main northern artery to Aleppo."

Assad's core forces, drawn mainly from his Alawite sect, are entrenched in the capital.